Option Explicit
Private Declare Function SendMessageBynum Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
Const LB_SETHORIZONTALEXTENT = &H194
Private Sub Command1_Click()
SendMessageBynum List1.hwnd, LB_SETHORIZONTALEXTENT, 300, 0
End Sub